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United States Patent |
6,197,465
|
Matsuzaki
,   et al.
|
March 6, 2001
|
Carrier for electrophotography and developer for electrophotography using
the carrier
Abstract
A carrier for electrophotography includes a carrier core material provided
with magnetism and a coating layer which coats the surface of the carrier
core material and includes a high molecular weight polyethylene resin
having a weight average molecular weight of 50,000 or more. An outermost
layer containing a magnetic powder having a three-dimensional form of a
convex polyhedron is formed on the outermost surface of the coating layer.
Inventors:
|
Matsuzaki; Shigeo (Sodegaura, JP);
Arakane; Takashi (Sodegaura, JP);
Murakata; Kazuo (Sodegaura, JP);
Kikuchi; Susumu (Tokyo, JP);
Mukataka; Hisashi (Watarai-gun, JP);
Kamiyama; Yuji (Watarai-gun, JP)
|
Assignee:
|
Idemitsu Kosan Co., Ltd. (Tokyo, JP);
Kyocera Corporation (Kyoto, JP)
|
Appl. No.:
|
426647 |
Filed:
|
October 25, 1999 |
Foreign Application Priority Data
| Dec 11, 1996[JP] | 8-330757 |
| Oct 30, 1998[JP] | 10-325992 |
Current U.S. Class: |
430/111.35; 430/111.41; 430/137.13 |
Intern'l Class: |
G03G 009/107 |
Field of Search: |
430/106.6,108,137
|
References Cited
U.S. Patent Documents
5079124 | Jan., 1992 | Kawata et al. | 430/108.
|
5085963 | Feb., 1992 | Suzuki et al. | 430/106.
|
5093201 | Mar., 1992 | Ohtani et al. | 428/407.
|
5166027 | Nov., 1992 | Machida et al. | 430/106.
|
5252398 | Oct., 1993 | Ohtani et al. | 428/403.
|
5272037 | Dec., 1993 | Ohtani et al. | 430/108.
|
5641601 | Jun., 1997 | Mitani et al. | 430/106.
|
5652060 | Jul., 1997 | Uchida et al. | 428/404.
|
5919593 | Jul., 1999 | Karima et al. | 430/108.
|
5968699 | Oct., 1999 | Matsuzaki et al. | 430/106.
|
Foreign Patent Documents |
10-171168 | Jun., 1998 | JP.
| |
Primary Examiner: Goodrow; John
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier & Neustadt, P.C.
Parent Case Text
This application is a Continuation-in-part (CIP) of application Ser. No.
09/147,033 filed on Sep. 15, 1998, now abandoned which was filed as an
International Application PCT/JP97/04563 on Dec. 11, 1997.
Claims
What is claimed is:
1. A carrier for electrophotography comprising a carrier core material
provided with magnetism and a coating layer which coats the surface of the
carrier core material and comprises a high molecular weight polyethylene
resin having a weight average molecular weight of 50,000 or more, wherein
an outermost layer containing a magnetic powder having a three-dimensional
form of a convex polyhedron is formed on the outermost surface of the
coating layer.
2. A carrier for electrophotography according to claim 1, wherein the
average particle diameter of the magnetic powder is in a range from 0.1 to
1 .mu.m.
3. A carrier for electrophotography according to claim 1, wherein the
surface of the magnetic powder is treated hydrophobically.
4. A carrier for electrophotography according to claim 1, wherein the
magnetic powder is embedded in the coating layer to form the outermost
layer and the surface polyethylene coating layer is formed so as to coat
the magnetic powder.
5. A carrier for electrophotography according to claim 1, wherein the
thickness of the surface polyethylene coating layer is in a range from
0.005 to 0.08 .mu.m.
6. A carrier for electrophotography according to claim 1, wherein at least
one material selected from the group consisting of carbon black, silica
and a charging characteristic resin is compounded in the coating layer.
7. A carrier for electrophotography according to claim 1, wherein the
volumetric resistance of the carrier for electrophotography is in a range
from 1.times.10.sup.2 to 1.times.10.sup.14 .OMEGA..cm.
8. A carrier for electrophotography according to claim 1, wherein the
average particle size of the carrier for electrophotography is in a range
from 20 to 120 .mu.m.
9. A process for producing a carrier for electrophotography comprising a
carrier core material provided with magnetism and a coating layer which
coats the surface of the carrier core material and comprises a high
molecular weight polyethylene resin having a weight average molecular
weight of 50,000 or more, the process comprising forming the coating layer
by a direct polymerization method and thereafter embedding a magnetic
powder, having a three-dimensional form of a convex polyhedron, in the
coating layer by a mechanical impact to form an outermost layer.
10. A process for producing a carrier for electrophotography according to
claim 9, wherein the magnetic powder having a three-dimensional form of a
convex polyhedron is embedded in the coating layer at a temperature
ranging between 50 and 120.degree. C. by a mechanical impact to form the
outermost layer and the surface polyethylene coating layer is formed so as
to coat the magnetic powder.
11. A developer for electrophotography comprising the carrier for
electrophotography according to claim 1 and a toner, wherein the mixing
ratio of the carrier for electrophotography is in a range from 2 to 40% by
weight to the total amount of the carrier and the toner.
Description
TECHNICAL FIELD
The present invention relates to a carrier for electrophotography and a
developer for electrophotography using the carrier. More particularly it
relates to a carrier for electrophotography used in development of an
electrostatic latent image in image formation using electrophotography,
and to a developer using the carrier.
BACKGROUND OF THE INVENTION
As an electrostatic latent image development for electrophotography,
one-component magnetic jumping development, one-component non-magnetic
contacting development, and two-component development, in which
development is performed by frictionally electrifying a toner,
transporting a developer, and contacting with an electrostatic latent
image, by mixing an insulating non-magnetic toner and magnetic carrier
particles, are known so far.
Particularly attention has been paid gain to application of the
two-component development to the color printer as a promising method in
near future.
A particulate carrier, which is used in such two-component development, is
usually produced by coating a magnetic carrier core material with an
appropriate material in order to prevent filming a toner onto the surface
of the carrier, to form a carrier-uniform surface, to elongate the
lifetime of a developer, to prevent damage of a sensitizer by a carrier,
to control charge quantity, or for other purposes.
Conventional resin-coated carriers are not, however, satisfactory in
durability because the coat is easily exfoliated by an impact such as
stirring applied when used or for other reasons.
To solve this problem, the inventors developed and proposed a method to
form a polyolefin-based resin coat by directly carrying out polymerization
of an olefin-based monomer on carrier-core-material particles of materials
such as ferrite, as described, for example, in Japanese Patent Laid-open
Pub. No. Hei. 2-187771. The polyolefin-based resin-coated carrier obtained
according to this method, 1) has the strong adhesion strength between the
core material and the coat, 2) does not give any deterioration in the
quality of image, 3) is excellent in durability, and 4) is excellent in
spent stability, even if copying is repeated continuously for a long time,
because the coat is directly formed on the carrier core material
particles.
On the other hand, however, this polyolefin-based resin-coated carrier did
not have adequate durability, not only because control of charge polarity
and adjustment of charge quantity can not be freely conducted, but also
because of the problem that the external additives are spent by attachment
of external additives of a toner or for other reasons.
In addition, the carrier did not have enough properties which allow fine
adjustment of resistance and adjustment of image density.
As methods to solve the above-mentioned problems, a method to improve
charge quantity by containing nigrosin in a carrier-coated resin is
disclosed in Japanese Patent Laid-open Pub. No. Sho. 53-100242, a method
to improve fluidity by adding a fluidity-improving agent is disclosed in
Japanese Patent Laid-open Pub. No. Sho. 61-9661, and a method, to prevent
making the charging property uniform and being spent by adding one
selected from a group consisting of electroconductive fine particles,
inorganic filler particles, and a charge-controlling agent, is disclosed
in Japanese Patent Laid-open Pub. No. Hei. 2-210365.
These methods, however, could not satisfy both 1) freely controlling charge
polarity, adjusting charge quantity, and adjusting resistance, with taking
advantage of an excellent property that the above-mentioned
polyolefin-based resin-coated carrier has, and 2) preventing external
additives of a toner from being spent.
While, the carriers produced by coating magnetic particles with a resin are
disclosed in U.S. Pat. Nos. 5,079,124, 5,085,963 and 5,652,060. All of
these carriers, however, use globular magnetic particles as the carrier
core material provided with magnetism which is used in the present
invention. These carriers not only nonuse convex polyhedron magnetic
particles but also are provided with no outermost layer containing convex
polyhedron magnetic particles. The object of the present invention in
which the convex polyhedron magnetic particles are added to the outermost
layer to secure easy control of charge quantity is unattainable by the
technologies disclosed in these patent publications.
In U.S. Pat. No. 5,641,601, a toner provided with convex polyhedron
magnetic particles stuck to the surface thereof is disclosed. However,
this toner is not used for a carrier. Also, even if this toner is combined
with the invention disclosed in U.S. Pat. No. 5,079,124, the structure of
the invention cannot be attained. Hence, the object of the present
invention to secure easy control of charge quantity is unattainable by the
technologies disclosed in U.S. Pat. No. 5,641,601.
The present invention aims to solve the above-mentioned problems, i.e. the
purpose of the present invention is to provide a carrier for
electrophotography and a developer using the carrier, which allows
adjusting the charge quantity and static resistance freely, with taking
advantage of an excellent property that a carrier having a
polyolefin-based resin coat has, obtaining an image stable in light and
shade, and effectively preventing external additives from being spent by
attachment of external additives of a toner.
DISCLOSURE OF THE INVENTION
The above object can be attained by the provision of a carrier for
electrophotography according to the present invention comprising a carrier
core material provided with magnetism and a coating layer (a covering
layer) which coats the surface of the carrier core material and comprises
a high molecular weight polyethylene resin having a weight average
molecular weight of 50,000 or more, wherein an outermost layer containing
a magnetic powder having a three-dimensional form of a convex polyhedron
is formed on the outermost surface (outer surface) of the coating layer.
In the structure of the carrier for electrophotography according to the
present invention, preferably the magnetic powder having a
three-dimensional form of a convex polyhedron is embedded in the coating
layer and the surface polyethylene coating layer is formed so as to coat
the magnetic powder.
According to another aspect of the present invention, there is provided a
process for producing a carrier for electrophotography comprising a
carrier core material provided with magnetism and a coating layer which
coats the surface of the carrier core material and comprises a high
molecular weight polyethylene resin having a weight average molecular
weight of 50,000 or more, the process comprising forming the coating layer
by a direct polymerization method and thereafter embedding a magnetic
powder, having a three-dimensional form of a convex polyhedron, in the
coating layer by a mechanical impact to form an outermost layer.
In the process for producing the carrier for electrophotography according
to the present invention, preferably the magnetic powder having a
three-dimensional form of a convex polyhedron is embedded in the coating
layer at a temperature ranging between 50 and 120.degree. C. by a
mechanical impact to form an outermost layer and the surface polyethylene
coating layer is formed so as to coat the magnetic powder.
According to a further aspect of the present invention, there is provided a
developer for electrophotography comprising the above carrier for
electrophotography and a toner, wherein the mixing ratio of the carrier
for electrophotography is in a range from 2 to 40% by weight to the total
amount of the carrier and the toner.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a view showing the relation between treating temperature and the
thickness of a surface polyethylene layer.
FIG. 2 is a view showing the relation between the thickness of a surface
polyethylene layer and a variation in charge quantity.
FIG. 3 is a view for explaining the magnet roller bias potential dependency
of an image density in Applied Example 1 of the present invention.
FIG. 4 is a view showing the results of evaluation of continuous printing
in Applied Example 2 of the present invention.
FIG. 5 is a view showing the relation between the number of printed amount
(copies) and image density.
FIG. 6 is a view showing the relation between bias potential (voltage) and
image density.
FIG. 7 is a photograph taken by a scan-type electron microscope showing the
surface of a carrier obtained in Example 6.
FIG. 8 is a photograph taken by a scan-type electron microscope showing the
entire of the carrier obtained in Example 6.
FIG. 9 is a photograph taken by a scan-type electron microscope showing the
surface of a carrier obtained in Example 14.
FIG. 10 is a sectional view of a carrier of the present invention which is
provided with a surface polyethylene coating layer.
FIG. 11 is a sectional view of a carrier of the present invention which is
not provided with a surface polyethylene coating layer.
BEST MODE FOR CARRYING OUT THE INVENTION
The embodiments of a carrier for electrophotography and a developer for
electrophotography using the carrier according to the present invention
will be explained concretely below.
I. Carrier for electrophotography
The carrier for electrophotography according to the present invention has a
carrier core material and a coating layer (a covering layer) consisting of
a high-molecular-weight polyethylene resin coating the surface of the
carrier core material, wherein said coating layer consisting of a
high-molecular-weight polyethylene resin contains a layer containing
magnetic powder that is a convex polyhedron that is encompassed by both or
either at least six flat and curved planes in the three-dimensional
geometry at least as its outermost layer, or a layer containing the above
magnetic powder and silica, or a layer containing the above magnetic
powder, silica and a fine particle resin.
Each component will be explained concretely below.
1. Carrier core material
(1) Material
There is no particular limitation to the core material of carrier according
to the present invention. Well known materials for the two
component-system carrier for electrophotography can be used, such as 1)
ferrite, magnetite, or the like; metals such as iron, nickel, and cobalt,
2) an alloy or a mixture of these metals with a metal such as copper,
zinc, antimony, aluminum, lead, tin, bismuth, beryllium, manganese,
magnesium, selenium, tungsten, zirconium, and vanadium, 3) a mixture of
the above-mentioned ferrite or the like with a metal oxide such as iron
oxide, titanium oxide, and magnesium, a nitride such as chromium nitride
and vanadium nitride; a carbide such as silicon carbide and tungsten
carbide, and 4) ferromagnetic ferrite, and 5) a mixture of these.
(2) Geometry and particle size
There is no particular limitation to the geometry of the carrier core
material. Both or either spherical and irregular forms are acceptable.
Although there is no particular limitation to an average particle size,
the average particle size of 20-100 .mu.m is preferable. If the average
particle size is smaller than 20 .mu.m, attachment (scattering) of the
carrier to the electrostatic latent image carrier (a sensitizer in
general) may occur. If the average particle size is larger than 100 .mu.m,
troubles such as carrier streaks may occur and cause deterioration of the
quality of image.
(3) Ratio of formulation
The weight ratio of the carrier core material per the overall carrier is
set to 90 wt. % or higher, preferably to 95 wt. % or higher. The ratio of
formulation indirectly specify the thickness of the resin-coated layer of
the carrier. If the weight ratio is lower than 90 wt. %, there may be the
case that the coating layer becomes too thick, and the durability and the
stability of charge which are required for a developer might not be
satisfied because of exfoliation of the coating layer, increase in the
charge quantity, and other reasons, even if the carrier is practically
applied to a developer. Also it may cause troubles such as low
reproducibility in fine lines and decrease in image density with respect
to the quality of image. Although there is no particular limitation to the
upper limit, such a ratio may be enough that the coated resin layer can
completely coat the surface of the carrier core material. This value
depends on the physical properties of the carrier core material and the
method for coating.
(4) Electroconductive layer
An electroconductive layer can be formed on the carrier core material
particles prior to coating with a high-molecular-weight polyethylene resin
if necessary.
As the electroconductive layer formed on the carrier core material
particles, for example, one, in which electroconductive fine particles are
dispersed in an appropriate binding resin, is favorable. The formation of
such an electroconductive layer is effective in enhancing a developing
property and obtaining clear images having high image density and clear
contrast. The reason for this is considered that the existence of the
electroconductive layer lowers electroresistance of the carrier to a
suitable level to balance leak and accumulation of electric charge.
As the electroconductive fine particle added to the electroconductive
layer, the followings can be used: carbon black such as carbon black and
acetylene black, carbide such as SiC, magnetic powder such as magnetite,
SnO.sub.2, and titanium black. As the binding resin of the
electroconductive layer, the followings can be used: various thermoplastic
resins and thermosetting resins such as polystyrene-based resins,
poly(metha)acrylic acid-based resins, polyolefin-based resins,
polyamide-based resins, polycarbonate-based resins, polyether-based
resins, polysulfonic acid-based resins, polyester-based resins,
epoxy-based resins, polybutyral-based resins, urea-based resins,
urethane/urea-based resins, silicone-based resins, and
Teflon(trademark)-based resins, and a mixture, a copolymer, a block
polymer, a graft polymer, and a polymer blend of these resins.
The electroconductive layer can be formed by coating a liquid in which the
above-mentioned electroconductive fine particles are dispersed in the
above-mentioned appropriate binding resin onto the surface of the carrier
core material particles by a method such as the spray coating method and
the dipping method. In addition, it can also be formed by
melting/blending/crushing the core material particles, electroconductive
fine particles, and a binding resin. In addition, it can also be formed by
polymerizing a polymerizable monomer on the surface of the core material
particle in the presence of the electroconductive fine particles. Although
there is no particular limitation to factors such as the size and the
amount of addition of the above-mentioned electroconductive fine particles
as long as the properties such as electroresistance of the carrier
according to the present invention are satisfied, an average particle size
of the electroconductive fine particle should be one that allows
homogeneous dispersion in the above-mentioned resin solution: concretely
0.01-2 .mu.m, preferably 0.01-1 .mu.m. Although the amount of the
electroconductive fine particles to add also depends on the kind and other
factors and it is not possible to specify it, a weight ratio of 0.1-60 wt.
% per the binding resin of the electroconductive layer, preferably 0.1-40
wt. % would be acceptable. Although such a trouble occurs that the
reproducibility decreases when fine lines are copied repeatedly using a
carrier like this when the packing ratio of the carrier is as small as ca.
90 wt. % and the thickness of the coating layer is relatively thick, this
kind of trouble can be dissolved by adding the above-mentioned
electroconductive fine particles.
The carrier core material particles on which a functional layer such as an
electroconductive layer was formed will also be designated hereafter
simply as "carrier core material particles" as long as misunderstanding
can be avoided.
2. Coating layer consisting of a high-molecular-weight polyethylene resin
(1) Molecular weight of a resin
High-molecular-weight polyethylene resins, which are usually designated as
"polyethylene", having a number-average molecular weight of 10,000 or
higher or a weight-average molecular weight of 50,000 or higher are
preferably used in the present invention. The followings waxes having a
number-average molecular weight lower than 10,000 are generally excluded
from the high-molecular-weight polyethylene resins for the present
invention: polyethylene wax (Mitsui High Wax, manufactured by Mitsui
Petrochemical Industries, Ltd.), Dialene 30 (manufactured by Mitsubishi
Gas Chemical Co., Ltd.), Nisseki Lexpole (manufactured by Nippon Oil Co.,
Ltd.), San Wax (manufactured by Sanyo Chemical Co., Ltd.), Polyrez
(neutral wax, manufactured by Polymer Co., Ltd.), Neowax (manufactured by
Yasuhara Chemical Co., Ltd.), AC Polyethylene (manufactured by Allied
Chemical Inc.), Eporene (manufactured by Eastman Kodak Co.), Hoechst Wax
(manufactured by Hoechst Co., Ltd.), A-Wax (manufactured by BASF Co.,
Ltd.), Polywax (manufactured by Petrolite Co., Ltd.), Escomer
(manufactured by Exxon Chemical Co., Ltd.), or the like. The polyethylene
wax may be coated by the conventional dipping method and the spray method
by dissolving in hot toluene or the like. However, since the mechanical
strength of the polyethylene wax is weak, it is exfoliated by the shear in
a developing machine after a long-term use or for other reasons.
It is also acceptable to add at least one kind of functional particles such
as the above-mentioned electroconductive fine particles and particles
having an ability to control the electric charge, which will be described
later, into the coating layer consisting of the above-mentioned
high-molecular-weight polyethylene resin.
(2) Method for forming coating layer
There is no particular limitation to form a coating layer used in the
present invention. Although well known methods such as the dipping method,
the fluidized bed method, the dry-type method, and the spray dry method
can be applied, the following polymerization method is preferred to coat
the polyethylene-based resin because the resin-coating strength is strong
and the coat is not be exfoliated easily.
a. Polymerization method
"The polymerization method" is a method to produce a polyethylene
resin-coated carrier by treating the surface of the carrier core material
with an ethylene-polymerizing catalyst and directly polymerizing ethylene
(forming polyethylene) on the surface, as described, for example, in
Japanese Patent Laid-open Pub. No. Sho. 60-106808 and Japanese Patent
Laid-open Pub. No. Hei. 2-187770. The polyethylene resin-coated layer can
be formed by suspending a product that is obtained in advance by
contacting a highly active catalytic component that contains both or
either titanium and zirconium, and is soluble in a hydrocarbon solvent,
such as hexane and heptane, with the carrier core material, and an
organoaluminum compound in the above-mentioned hydrocarbon solvent,
supplying an ethylene monomer, and polymerizing it on the surface of the
carrier core material. In addition, in case fine particles or
electroconductive fine particles having the above-mentioned an electric
charge-conferring function are added, they can be added while the
above-mentioned high-molecular-weight polyethylene resin-coated layer is
formed.
As this production forms a polyethylene-coated layer directly on the
surface of the carrier core material, a coat excellent in strength and
durability is obtained.
If functional fine particles such as electroconductive fine particles and
fine particles having an ability to control electric charge are
dispersed/coexisted in the polymerization system in this way, while a
high-molecular-weight polyethylene resin coat is growing/being formed by
polymerization, the functional fine particles are incorporated into this
coat, and a high-molecular-weight polyethylene resin coat containing the
functional particles is formed.
b. Amount of coating
A high-molecular-weight polyethylene resin coat is formed with a weight
ratio of [carrier core material]/[high-molecular-weight polyethylene resin
coat] being preferably in a range of 99.5/0.5-90/10, more preferably in a
range of 99/1-95/5.
c. Addition and supporting of functional fine particles
The quality of the carrier can be improved, as described above, by
adding/carrying at least one kind of functional particles, such as
electroconductive fine particles and particles having an ability to
control electric charge, in the high-molecular-weight polyethylene resin
coat.
As electroconductive fine particles which are added/carried in
high-molecular-weight polyethylene resin coat, can be used all well-known
ones, for example, carbide such as carbon black and SiC, electroconductive
magnetic powder such as magnetite, SnO.sub.2, titanium black. A preferable
average particle size of the electroconductive fine particles is 0.01-5.0
.mu.m.
(3) Outermost layer
The coating layer has a layer containing magnetic powder that is a convex
polyhedron that is encompassed by at least six flat and/or curved planes
in the three-dimensional geometry at least as its outermost layer, or a
layer containing said magnetic powder and silica and/or fine particle
resin. It is noted that the layer containing said magnetic powder and
silica and/or fine particle resin is called as its outermost layer in the
case that the later-described surface polyethylene is formed on the
outermost layer.
a. Magnetic powder
Magnetite, ferrite, iron powder, or the like can be used as a material for
the magnetic powder used in the present invention.
The three-dimensional geometry of the magnetic powder is a convex
polyhedron that is encompassed by both or either at least six flat and
curved planes. Although "polyhedron" usually means a steric body that is
encompassed only with flat planes, "polyhedron" in the present invention
is referred to as a solid body that is encompassed with both or either
flat and curved planes. The existence of angles and vertices formed by
flat and curved planes like this is important.
If the carrier is a polyhedron like this, as the electroconduction changes
from the surface-electroconduction mechanism to the
point-electroconduction mechanism in a convex part of a polyhedron, the
efficiency in electroconduction is improved. For the polyhedron, both a
single kind and a combination of a plurality of kinds are acceptable.
The average particle diameter (size) is preferably 0.1-1 .mu.m, more
preferably 0.2-0.7 .mu.m. If the diameter is smaller than 0.1 .mu.m, the
effect as a spacer might be lost. If the diameter is larger than 1 .mu.m,
there may be the case where the addition to its outermost layer becomes
impossible.
The resistance is preferably
1E+7(1.times.10.sup.7)-1E+10(1.times.10.sup.10) .OMEGA..cm, more
preferably 1E+7(1.times.10.sup.7)-1E+9(1.times.10.sup.9) .OMEGA..cm. If
the resistance is smaller than 1E+7 .OMEGA..cm, a charge property might be
prevented. If the resistance is larger than 1E+10 .OMEGA..cm, adjustment
of the resistance might become impossible, and the function as a magnetic
powder might not be performed.
They are commercially available, for example, from Mitsui Metal Co. as
Magnetite MG-1306 (octahedron, the average particle diameter 0.2 .mu.m)
and Magnetite MG-3900 (polyhedron, the average particle diameter 0.2
.mu.m).
b. Silica
Silica, whose surface was hydrophobically treated and positively or
negatively charged, can be used in the present invention.
The particle size is preferably equal to or smaller than 40 nm in primary
size, more preferably 10-30 nm. If the size is larger than 40 nm, gaps
between silica particles may become large and ruggedness is generated on
the surface of the carrier.
As positively charged silica, for example, RA200HS (manufactured by Nippon
Aerosol Co., Ltd.), 2015EP, and 2050EP (both Wacker Chemicals Co., Ltd.)
are commercially available. As negatively charged silica, for example,
R812, RY200 (both manufactured by Nippon Aerosol Co., Ltd.), 2000, and
2000/4 (both Wacker Chemicals Co., LTD) are commercially available.
It is preferable to add negatively charged silica to a positively charged
toner, and to add positively charged silica to a negatively charged toner.
c. Fine particle resin
The following negatively charged resins (A) and positively charged resins
(B) can be used as the fine particle resin according to the present
invention.
A. Negatively charged resins
Fluorine-based resin (such as a fluorovinylidene resin, a
tetrafluoroethylene resin, a trifluorochloroethylene resin, and a
tetrafluoroethylene/hexafluoroethylene copolymer resin), a vinyl
chloride-based resin, and celluloid.
B. Positively charged resins
An acryl resin, a polyamide-based resin (such as nylon-6, nylon-66, and
nylon-11), a stylene-based resin (polystylene, ABS, AS, and AAS), a
chlorovinylidene resin, a polyester-based resin (such as polyethylene
terephthalate, polyethylene naphthalate, polybutylene terephthalate,
polyacrylate, polyoxybenzoyl, and polycarbonate), a polyether-based resin
(such as polyacetal and polyphenylene ether), and an ethylene-based resin
(such as EVE, EEA, EAA, EMAA, EAAM, and EMMA).
It is preferable to add a negatively charged resin to a positively charged
toner, and to add a positively charged resin to a negatively charged
toner.
It is acceptable to contain both above-mentioned silica and particle resin
as well as to contain one of them. In addition, one kind or a plurality of
kinds of silica can be used, and one kind or a plurality of kinds of
particle resin(s) can be used.
d. Coat thickness
The coat thickness of its outermost layer is preferably 0.1-6 .mu.m. If it
is thinner than 0.1 .mu.m, coating might become incomplete. If it is
thicker than 6 .mu.m, its outermost layer might be exfoliated by a
mechanical impact such as friction from the outside.
e. Formation and fixing method of outermost layer
Formation and fixing method of outermost layer used in the present
invention can be performed, depending on particle size and geometry of the
magnetic powder to use and physical properties, such as particle size,
solubility to organic solvents, melting point, and hardness, of silica
and/or a resin, by selecting a method from the following two methods or by
combining them.
(i) Fixing by mechanical impact
A Henshel mixer or the like, such as a 20C/I-type Henshel mixer
manufactured by Mitsui Miike Chemical Machine Co. , Ltd. is preferably
used to perform formation and fixing by a mechanical impact.
Here, although there is the case where the degree of the mechanical impact
is altered corresponding to the amount of the coating (the amount of
polyethylene) in the carrier core material, the amount of the magnetic
powder or the amount of silica particles and microparticle resin used in
combination with the magnetic powder, it is generally desirable that one
throughput be designed to be in a range between 3 and 20 kg and the number
of revolutions be designed to be in a range between 200 and 3000 rpm.
The treating temperature when the mechanical impact is applied is in a
range between preferably 50 and 120.degree. C., more preferably 60 and
110.degree. C. and most preferably 70 and 100.degree. C. This is because
when treating temperature is less than 50.degree. C., embedment of the
magnetic powder in the coating layer tends to be difficult and there is
the case where a surface polyethylene coating layer with appropriate
thickness is not formed. Hence there is the case where the charging
characteristics are greatly changed in a high temperature and high
humidity condition.
On the contrary, when the temperature exceeds 120.degree. C., there is the
case where the carriers are deposited among themselves to produce excess
carriers and to make the thickness of the surface polyethylene coating
layer too large, resulting in a large change in the charging
characteristics in a low temperature and low humidity condition.
The above point will be explained in more detail with reference to FIGS. 1
and 2. FIG. 1 is a view showing the relation between treating temperature
and the thickness of a surface polyethylene layer and FIG. 2 is a view
showing the relation between the thickness of a surface polyethylene layer
and a variation in charge quantity.
FIG. 1 is made based on the data obtained in Examples 6 to 12, wherein the
abscissa indicates treating temperature (.degree. C.) and the ordinate
indicates the thickness (.mu.m) of a surface polyethylene coating layer.
As is understood from FIG. 1, the treating temperature closely relates to
the thickness of the surface polyethylene coating layer. There is a
tendency that the thickness of the formed surface polyethylene coating
layer increases with an increase in treating temperature. There is also
observed a tendency that the thickness of the surface polyethylene coating
layer becomes remarkably thick at a temperature around 100 (.degree. C.)
or more.
FIG. 2 is made based on the data obtained in Examples 6 to 12 and in
Comparative Example 1, wherein the abscissa indicates the thickness
(.mu.m) of surface polyethylene coating layer and the ordinate indicates a
difference in charge quantity (.mu.C/g). Incidentally, the solid line A
shows a difference in charge quantity (hereinafter noted as a difference
LL-NN in charge quantity) calculated by subtracting the value of charge
quantity (NN charge quantity) measured under a normal temperature and
normal humidity circumstance from the value of charge quantity (LL charge
quantity) measured under a low temperature and low humidity circumstance.
The dot line B, in turn, shows a difference in charge quantity
(hereinafter noted as a difference NN-HH in charge quantity) calculated by
subtracting the value of charge quantity (HH charge quantity) measured
under a high temperature and high humidity circumstance from the value of
charge quantity (NN charge quantity) measured under a normal temperature
and normal humidity circumstance.
As is understood from FIG. 2, a difference in charge quantity tends to
increase when the thickness of the surface polyethylene coating layer is
too large or too small. When the thickness of the surface polyethylene
coating layer is too large, the value of the difference LL-NN in charge
quantity tends to increase as shown by the solid line A. When the
thickness of the surface polyethylene coating layer is too small, the
value of the difference NN-HH in charge quantity tends to increase as
shown by the dot line B.
Hence, the thickness of the surface polyethylene coating layer to be formed
can be limited to the value falling within a prescribed range, for
example, 0.005 to 0.08 .mu.m by applying a mechanical impact at
appropriate treating temperatures. This makes it possible to reduce a
change in the charging characteristics of the carrier with a change in
ambient conditions, specifically to reduce it to 2 .mu.C/g or less,
preferably 1 .mu.C/g or less and more preferably 0.5 .mu.C/g or less.
It is desirable that the treating time in the application of a mechanical
impact should be generally 0.5 to 6 hours though there is the case where
it is altered corresponding to the amount of the coating (the amount of
polyethylene) in the carrier core material, the amount of the magnetic
powder or the amount of silica particles and microparticle resin used in
combination with the magnetic powder.
When a mechanical impact is applied using a Henshel mixer, there is the
case where a magnetic powder and the like remain unburied and it is hence
preferable to carry out sufficient screening and classification after
mechanical impact treatment.
(ii) Thermal fixation by heating
Its outermost layer is formed by mixing the high-molecular-weight
polyethylene resin-coated carrier and an appropriate amount of magnetic
powder or a mixture prepared by mixing the magnetic powder and both or
either silica and fine particle resin using a machine, which can heat,
such as the Thermal Spheronizing Machine (Hosokawa Micron Co., Ltd.). The
amount of magnetic powder and the amount of silica and/or fine particle
resin to add then are determined by absolute value of charge quantity to
change and stability of real printing image density.
Although, concretely, it is usual to add at a weight ratio of 0.1-50 phr
(external additives per coating resin) per 100 phr of the coating
polyethylene of a high-molecular-weight polyethylene-coated carrier, an
appropriate ratio is 20-30 phr, considering durability and change in
resistance accompanying the formation of its outermost layer, production
stability.
In the thermal spheronization treatment, it is necessary to uniformly
attach magnetic powder and both or either silica and a fine particle resin
to the surface of the high-molecular-weight polyethylene resin-coated
carrier before the treatment. For this purpose, a mixing treatment such as
the ball-mill treatment, the V-blender treatment, and the Henshel-mixer
treatment (for ca. 1 min) is carried out to electrostatically or
mechanically attach the particles of magnetic powder and both or either
silica and fine particle resin onto the surface of the
high-molecular-weight polyethylene resin-coated carrier. Fixing was
performed and an outermost layer is formed by heating for a very short
time with uniformly attaching onto the surface of the
high-molecular-weight polyethylene resin-coated carrier.
f. Surface polyethylene coating layer
In the structure of the carrier for electrophotography as shown in FIG. 10,
preferably a coating layer 14 which coats a carrier core material 12
having magnetism and an outermost layer 18 in which a magnetic powder 16
having a convex polyhedron is embedded are formed on the surface of the
carrier core material 12 and a surface polyethylene coating layer (a
surface polyethylene covering layer) 20 is further formed on the surface
of the outermost layer 18 so as to cover the magnetic powder 16.
The section of a carrier 22 provided with no surface polyethylene coating
layer is also shown in FIG. 11 for the sake of understanding of the
surface polyethylene coating layer 20.
When the carrier is structured such that the magnetic powder is not exposed
as shown in FIG. 10, absorption of water in air can be efficiently
prevented because the magnetic powder having a convex polyhedron form is
not in direct contact with air. Therefore, the influence of water in air,
namely, the effect of moisture is overcome to attain better control of
charging characteristics by the carrier.
The thickness of the surface polyethylene coating layer is preferably in a
range from 0.005 to 0.08 .mu.m. This is because when the thickness of the
surface polyethylene coating layer is less than 0.005 .mu.m, there is the
case where the magnetic powder having a convex polyhedron form easily
absorbs water in air whereas when the thickness exceeds 0.08 .mu.m, there
is the case where the charging characteristics is controlled with
difficulty on the contrary.
Hence, the thickness of the surface polyethylene coating layer is in a
range from more preferably 0.008 to 0.05 .mu.m and most preferably 0.01 to
0.04 .mu.m.
It is noted that the aforementioned surface polyethylene coating layer may
be formed at the same time when the outermost layer in which the magnetic
powder having a convex polyhedron form is embedded is formed.
Alternatively, the surface polyethylene coating layer may be formed on the
surface of the outermost layer after the outermost layer has been formed.
For instance, in the case of forming the surface polyethylene coating layer
and the outermost layer simultaneously, the both layers can be formed with
accuracy by designing the treating time in the Henshel mixer to be 2 hours
or more, or the treating temperature to fall in a range between 50 and
120.degree. C. Specifically, the magnetic powder is embedded in the
polyethylene resin, applied to the carrier core material, to a relatively
great depth to separate a polyethylene resin layer as the outermost layer
including the magnetic powder from a polyethylene resin layer as the
surface polyethylene coating layer excluding the magnetic powder, thereby
forming the both layers simultaneously.
3. Electroconductive property of carrier
Although the optimal electroconductivity of a carrier depends on the system
of the developer in which the carrier is used, a carrier showing a value
of 1.times.10.sup.2 -1.times.10.sup.14 .OMEGA..cm is preferred in general.
If the value is lower than 1.times.10.sup.2 .OMEGA..cm, carrier development
and overlapping may occur. If the value is higher than 1.times.10.sup.14
.OMEGA..cm, deterioration in the quality of image such as lowering of the
image density may occur.
Resistance values were determined by placing a carrier layer having an
electrode area of 5 cm.sup.2, a load of 1 kg, and a thickness of 0.5 cm,
applying a voltage of 1-500 V to both upper and lower electrodes,
measuring current values flowing in the bottom, and converting the values.
4. Average particle size of carrier
Although the optimal average particle size of a carrier depends on the
system of the developer in which the carrier is used, for example, the
average particle size is preferably in a range from 20 to 120 .mu.m, more
preferably in a range from 20 to 100 .mu.m and the most preferably in a
range from 20 to 80 .mu.m.
If the average particle size is lower than 20 .mu.m, carrier development
may occur and the conveying ability of the carrier may be difficult. If
the average particle size is higher than 120 .mu.m, overlapping may occur.
II. Developer for electrophotography
The developer for electrophotography according to the present invention can
be obtained by mixing various toners with the above-mentioned carrier.
1. Toner
As a toner used in the present invention, the toner, which was produced
according to a well-known method such as the suspension polymerization
method, the crushing method, the microcapsule method, the spray dry
method, and the mechanochemical method, can be used, and at least binder
resins, coloring agents, and other additives such as electric
charge-controlling agents, lubricants, off-set-preventing agents, and
fixation-enhancing agents can be formulated, if necessary, to effectively
improve a developing property and prevent scattering of a toner in the
machine. In addition, fluidizing agents can also be added to improve its
fluidizability. Binder resins which can be used are polystylene-based
resins such as polystylene, stylene/butadiene copolymer, and stylene/acryl
copolymer; ethylene-based copolymers such as polyethylene, ethylene/vinyl
acetate copolymer, and ethylene/vinyl alcohol copolymer; epoxy-based
resins; phenol-based resins; acryl phthalate resin; polyamide resin;
polyester-based resins; and maleic acid resin. Coloring agents which can
be used are well known dyes/pigments such as carbon black, Copper
Phthalocyanine Blue, Indus MeliaBlue, Peacock Blue, Permanent Red, Red
Oxide, Alizarin Rake, Chrome Green, Malachite Green Rake, Methyl Violet
Rake, Hansa Yellow, Permanent Yellow, and titanium oxide. Electric
charge-controlling agents which can be used are positive electric
charge-controlling agents such as nigrosin, nigrosin base,
triphenylmethane-based compounds, polyvinylpyridine, and quaternary
ammonium salt; and negative electric charge-controlling agents such as
metal-complexes of alkyl-substituted salicylic acid (e.g. a chromium
complex or a zinc complex of di-tert-butylsalicylic acid). Lubricants
which can be used are Teflon (trademark, tetrafuluoroethylene), zinc
stearate, and polyfluorovinylidene. Off-set-preventing/fixation-enhancing
agents which can be used are a polyolefin wax or the like such as
low-molecular-weight polypropylene and its modification. Magnetic
materials which can be used are magnetite, ferrite, iron, and nickel.
Fluidizing agents which can be used are silica, titanium oxide, aluminum
oxide, or the like.
The average size of the toner is preferably equal to or lower than 20
.mu.m, more preferably 5-15 .mu.m.
2. Mixing ratio
The weight ratio of toner/carrier according to the present invention is
2-40 wt. %, preferably 3-30 wt. %, more preferably 4-25 wt. %. If the
ratio is lower than 2 wt. %, the toner charge quantity may become high,
and there may be the case where enough image density is not obtained. If
the ratio is higher than 40 wt. %, there may be the case where enough
charge quantity is not obtained, and the toner scatters from the
developing machine and pollutes inside the copying machine, or causes
toner-overlapping.
3. Usage
The developer according to the present invention is used in the 2- and
1.5-component-type electrophotography system such as the copying machine
(analogue, digital, monochrome, and color type), the printer (monochrome
and color type), and the facsimile, especially most suitably in the
high-speed/ultra-high-speed copying machine and printer or the like in
which the stress applied to the developer is high in the developing
machine. There is no particular limitation to the type of image-formation,
the type of exposure, the type (apparatus) of development, and various
types of control (e.g. the type of controlling the density of a toner in a
developing machine). One can adjust it to an optimal resistance, an
average particle diameter(size)/particle diameter(size) distribution, a
magnetic power, and an charge quantity of the carrier and the toner,
depending on the system.
EXAMPLES
The examples of the present invention will be described more concretely
below.
Production of Carrier
(1) Preparation of titanium-containing catalytic component
Into a 500-ml flask whose atmosphere was replaced for argon, 200 ml of
dried n-heptane and 15 g (25 mmol) of magnesium stearate that had been
dried at 120.degree. C. under a reduced pressure (2 mmHg) were added at
room temperature to make a slurry. After 0.44 g (2.3 mmol) of titanium
tetrachloride was dropwise added with stirring, the content began to be
heated, the reaction was carried out under reflux for 1 hour, and a clear
viscous solution of a titanium-containing catalyst (the active catalyst)
was obtained.
(2) Evaluation of activity of titanium-containing catalytic component
Into a 1-liter autoclave whose atmosphere was replaced for argon, 400 ml of
dried hexane, 0.8 mmol of triethylaluminum, 0.8 mmol of diethylaluminun
chloride, and 0.004 mmol (as titanium atom) of the titanium-containing
catalytic component obtained in (1) were added, and the content was heated
up to 90.degree. C., wherein the inner pressure of the system was 1.5
kg/cm.sup.2 G. After hydrogen was supplied up to 5.5 kg/cm.sup.2 G,
ethylene was continuously supplied maintaining the total pressure at 9.5
kg/cm.sup.2 G. Polymerization was carried out for 1 hour, giving 70 g of
polymer. The polymerization activity was 365 kg/g.Ti/Hr, and MFR (melt
flow rate at 190.degree. C., a loading of 2.16 kg according to JIS K 7210)
of the polymer obtained was 40 g/min.
(3) Production of polyethylene-coated carrier
Into a 2-liter autoclave whose atmosphere was replaced for argon, 960 g of
sintered ferrite powder F-300 (Powder Tech Corp., average particle size 50
.mu.m) was added, the content was heated up to 80.degree. C., and drying
was carried out under a reduced pressure (10 mmHg) for 1 hour. After the
content was cooled down to 40.degree. C., 800 ml of dried hexane was
added, and mixing was started. After 5.0 mmol of diethylaluminum chloride
and the titanium-containing catalytic component described in (1) (0.05
mmol as titanium atom) were added, reaction was carried out for 30 min.
Then, the content was heated up to 90.degree. C., 4 g of ethylene was
introduced, with the inner pressure being 3.0 kg/cm.sup.2 G. After
hydrogen was supplied up to 3.2 kg/cm.sup.2 G, 5.0 mmol of
triethylaluminum was added to start polymerization. The inner pressure of
the system went down to and was stabilized at 2.3 kg/cm.sup.2 G in ca. 5
min. Then, a slurry containing 5.5 g of carbon black (Mitsubishi Chem.
Co., MA-100) in 100 ml of dried hexane was added, polymerization was
carried out continuously supplying ethylene, with keeping the inner
pressure at 4.3 kg/cm.sup.2 G for 45 min (the supply was stopped when 40 g
of ethylene was introduced into the system), and 1005.5 g of carbon
black-containing polyethylene resin-coated ferrite was obtained. Died
powder of it was uniformly black. Electron-microscopic observation
revealed that the surface of the ferrite was coated with a thin
polyethylene layer and the carbon black is uniformly dispersed in the
polyethylene layer. Thermal gravimetric analysis (TGA) of the composite
revealed that the weight ratio of ferrite/carbon black/polyethylene was
95.5/0.5/4.0.
The intermediate-step carrier obtained through this step was designated as
"the carrier A.sub.1 ". The weight-average molecular weight of the coating
polyethylene was 206,000.
Then carrier A.sub.1 was classified using a sieve of 125 .mu.m to remove
particles which are equal to or larger than 125 .mu.m in diameter. The
carrier after the classification was added into a fluidized-bed type
gas-flow classifier having a height of 14 cm, and heated air (115.degree.
C.) was blown in to give at a linear velocity of 20 cm to fluidize the
carrier for 10 hours. Thus carrier A.sub.2 was obtained.
Example 1
Into a 10-liter Henshel mixer (Mitsui Miike Co., FM10L), 1000 g of the
carrier A.sub.2 was added and mixed for 1 hr to give mechanical impact and
to smoothen the surface of carrier A.sub.2. Then 8 g of magnetic powder
(Mitsui Metal Co., Magnetite MG1306, octahedron, the average particle
diameter of 0.2 .mu.m) was added and mixed for another 1 hr to give
mechanical impact to form outermost layer containing magnetic powder. To
remove magnetic powder existing unfixed freely, the large particle size
carrier and the aggregated magnetic powder were removed using a sieve. In
addition, to remove particles such as the unfixed magnetic powder,
treatment was carried out using a fluidized-bed type gas-flow classifier
at a linear velocity of 20 cm for 2 hours. Thus carrier B was obtained.
Example 2
Carrier C was obtained according to the same method as Example 1 except
that 20 g of magnetic powder was used instead of 8 g.
Example 3
Into a 10-liter Henshel mixer (Mitsui Miike Chemical Eng. Machine Co., Ltd.
FM10L), 1000 g of the carrier A.sub.2 and mixed for 1 hr to give
mechanical impact and to smoothen the surface of carrier A.sub.2. Then 8 g
of magnetic powder (manufactured by Mitsui Metal Co., LTD., Magnetite
MG1306, octahedron) was added and mixed for another 1 hr to give
mechanical impact, and 12 g of silica (manufactured by Nippon Aerosil Co.,
LTD., R812) was added and mixed for another 1 hr to give mechanical
impact, forming magnetic powder-silica-containing outermost layer. To
remove magnetic powder existing unfixed freely, the large particle size
carrier and the aggregated magnetic powder were removed using a sieve. In
addition, to remove particles such as the unfixed magnetic powder,
treatment was carried out using a fluidized-bed type gas-flow classifier
at a linear velocity of 20 cm for 2 hours. Thus carrier D was obtained.
Example 4
1000 g of the carrier A.sub.2 was placed into a 10-liter Henshel mixer
(Mitsui Miike Chemical Machine Co., Ltd. FM10L). Then 8 g of the magnetic
powder (manufactured by Mitsui Mining & Smeling Co., LTD., Magnetite
MG1306, octahedron) and 8 g of a micropowdered resin (manufactured by
Soken Kagaku Co., LTD., MP2701) were added and mixed for 1 minute, whereby
these were caused to adhere to the surface of the carrier A.sub.2
electrically and mechanically. Then, the mixture was heat-treated with
heating air at 200.degree. C. by using a thermal sphere forming machine
(manufactured by Hosokawa Micron Co., Ltd., Thermal Sphere Forming
Machine) to fix the magnetic powder and the micropowdered resin into the
melted coating polyethylene resin layer, whereby the outermost layer mixed
with the magnetic powder and the micropowdered resin was formed. The
carrier with a large diameter, coagulated magnetic powder and coagulated
micropowdered resin were removed by using sieve classification for the
purpose of eliminating excesses of the magnetic powder and micropowdered
resins existing free without being fixed. Also, the resultant product was
processed using a fluidized-bed type air flow classification at an air
flow linear velocity of 20 cm for two hours for the purpose of eliminating
excesses of the magnetic powder and micropowdered resins without being
fixed. Thus carrier E was obtained.
Example 5
Carrier F was obtained according to the same method as Example 1 except
that Magnetite MG9300 (manufactured by Mitsui Metal Co., Ltd., the average
particle diameter (size) of 0.2 .mu.m) was used instead of Magnetite
MG1306 (manufactured by Mitsui Mining & Smeling Co., Ltd.).
Comparative Example 1
Carrier A.sub.2 obtained in the carrier production example was not further
treated and evaluated in the same way of Example 1.
Comparative Example 2
Carrier G was obtained according to the same method as Example 1 except
that Magnetite MG8200 (manufactured by Mitsui Metal Co., sphere, the
average particle diameter (size) of 0.2 .mu.m) was used instead of
Magnetite MG1306 (manufactured by Mitsui Metal Co.).
Applied Example 1
Evaluation of real printing was carried out, using the toners A and B with
respect to each of the carriers A.sub.2 -G obtained in Examples 1-5 and
Comparative Examples 1 and 3, using a machine that was modified from
Ecosys 5 3550 (Kyocera Co.) in such a way that amorphous silicon was used
as a photoreceptor when a positively charged toner was evaluated, that an
organic electrophotography photoreceptor was used when a negatively
charged toner was evaluated, and that the surface potential of the
photoreceptor and the magnet roller bias potential could be adjusted.
Results of evaluation of real printing, charge quantity, and static
resistance are summarized in Table 1.
The followings were used as Toner A and Toner B:
Toner A:
Stylene/n-butylmethacrylate copolymer resin
100 wt. parts
Carbon black (Mitsubishi Chem. Co., MA#8)
5 wt. parts
Dye (Orient Chem. Ind. Co., N07)
5 wt. parts
Toner A was obtained by adequately mixing the above materials using a ball
mill, blending using three rolls heated at 140.degree. C., cooling the
mixture by standing, and roughly crushing using a feather mill, and
further finely crushing using a jet mill.
Toner B:
Bisphenol A-based polyester resin
100 wt. parts
Carbon black (Cabot Corp., BPL)
8 wt. parts
Dye (Orient Chem. Ind. Co., E-84)
5 wt. parts
Toner B was obtained by adequately mixing the above materials using a ball
mill, blending using three rolls heated at 140.degree. C., cooling the
mixture by standing, and roughly crushing using a feather mill, and
further finely crushing using a jet mill.
Evaluation of real printing was carried out by evaluating density of the
printed part by using the Macbeth densitometer after printing at various
bias potentials. Also static resistance and charge quantity were
simultaneously measured. Measurement of charge quantity was carried out
using a charge quantity-measuring device (Toshiba Chemical Co., Ltd.
TB-200). The measurement was carried out by mixing 0.5 g of a toner and
9.5 g of a carrier, putting the mixture into a 500-ml plastic bottle,
tumbling in a ball mill for 1 hr, at a blow pressure of 0.8 kg/cm.sup.2,
for a blowing time of 50 sec, using a 500-mesh stainless steel sieve.
TABLE 1
Charge quantity Static Image density
Kind of (.mu.C/g) Resistance (Measured at each bias potential)
carrier Toner A Toner B (.OMEGA. cm) 150 V 200 V 250 V 300 V
250 V
Carrier A.sub.2 +11.2 -13.5 3.1E + 11 1.17 1.23 1.30 1.32
l.33
Carrier B +10.9 -13.1 1.1E + 10 1.19 1.27 1.35 1.44 1.53
Carrier C +11.0 -12.9 8.9E + 08
Carrier D +18.2 -7.2 7.8E + 12
Carrier E +7.5 -19.3 6.3E + 13
Carrier F +11.2 -13.3 2.7E + 11
Carrier G +10.8 -13.4 2.5E + 10 1.21 1.24 1.29 1.32 1.34
Dependence of image density on magnet roller bias potential in Applied
Example 1 is illustrated in FIG. 1.
As it is clear from the description above, by making the geometry of
magnetic powder to add to an outermost layer of the carrier for
electrophotography, which is used as a developer for electrophotography,
octahedron or the like, the proportional relation between bias potential
and image density is obtained, the increasing rate of image density is
still high even at a high bias potential, and obtaining clear light and
shade in printing and stable image becomes possible.
Applied Example 2
Carriers B, A.sub.2, or G obtained in Example 1, Comparative Example 1, or
Comparative Example 3, respectively, was mixed with a toner (Kyocera
Corporation, Ecotone(trademarak)) at a toner concentration of 5 wt. %
(T/C), loaded into a Printer FS3550 (Kyocera Corporation, Ecosys
(trademarak)), and evaluation of continuous printing was carried out. The
result is summarized in FIG. 4.
Example 6
10 kg of the carrier A.sub.2 was placed in a Henshel mixer (FM2OC/I-model,
manufactured by Mitsui Miike Chemical machine Co., Ltd.) with a capacity
of 20 l. Hot water was flowed through a jacket formed around the Henshel
mixer to raise the temperature (treating temperature) in the Henshel mixer
to 70.degree. C. The Henshel mixer was allowed to work to agitate the
carrier for 0.5 hours while the temperature was kept at 70.degree. C.,
thereby applying a mechanical impact to the carrier to smooth the surface
of the carrier A.sub.2.
Then, 200 g of Magnetite MG1306 was mixed as the magnetic powder and
thereafter the Henshel mixer was operated for 3 hours to apply a
mechanical impact, thereby forming a magnetic powder-containing
polyethylene resin layer (outermost layer) and a surface polyethylene
coating layer simultaneously on the carrier A.sub.2. In addition, for the
purpose of removing magnetic powder which was not fixed and remained in a
free condition, though its amount was small, the resulting product was
treated in a screening process (#125 mesh) and a classifying process
(using a fluidized-bed air classifier, linear velocity: 20 cm, 2 hours) to
obtain a carrier H.
a. Observation by scanning electron microscope (SEM).
The surface of the resulting carrier was observed using an SEM. As a
result, it was confirmed that, as shown in FIG. 7, a magnetic powder
having a octahedron form was embedded in a polyethylene resin, which is a
coating layer, in the condition that a surface polyethylene coating layer
is formed. Therefore, it was confirmed that the surface polyethylene
coating layer was formed on the surface of the magnetic powder and the
magnetic powder having a octahedron form was not exposed from the surface.
It was also confirmed that the surface of the carrier H on which the
surface polyethylene coating layer was formed was extremely smooth and the
carrier H was entirely close to a true sphere.
b. Auger electron spectrometry
The thickness of the surface polyethylene coating layer formed on the
magnetic powder in the resulting carrier H was confirmed using an Auger
electron spectrometry apparatus JAMP-7100 (manufactured by JEOL). To state
in more detail, a scanning electron microscope was used to confirm the
position of the magnetic powder in the plane direction in advance and
argon (Ar.sup.+) sputtering and Auger electron spectrometry were repeated
to make a measuring chart (profile in the direction of thickness). The
time until an Fe element contained in the magnetic powder was detected was
calculated by converting from the ratio of the sputtering rates of a
polyethylene resin and SiO.sub.2 on the basis of the measuring chart to
calculate the thickness of the surface polyethylene coating layer. As a
consequence, it was confirmed that the thickness of the surface
polyethylene coating layer was 0.005 .mu.m.
c. Measurement of charge quantity
The resulting carriers H were measured for the charge quantity in the
following three conditions: a high temperature and high humidity condition
(HH condition, temperature: 33.degree. C., relative humidity: 85%), a
normal temperature and normal humidity condition (NN condition,
temperature: 25.degree. C., relative humidity: 60%) and a low temperature
and low humidity condition (LL condition, temperature: 10.degree. C.,
relative humidity: 20%). 9.5 g of each carrier H and 0.5 g of a toner
(TK-12 model, manufactured by Kyocera Co.) was placed in a plastic bottle,
allowed to stand for 48 hours in each condition and thereafter stirred for
one hour in a ball mill to charge the carrier forcibly. The carrier H and
the toner were taken out from the plastic bottle to measure the charge
quantity of the carrier H by using a charge quantity measuring apparatus
(TB-200 model, manufactured by Toshiba Chemical Co., Ltd.) in the
following condition: blowing pressure: 0.8 kg/cm.sup.2, blowing time: 50
seconds, using a 500 mesh stainless wire-gauge. The results are shown in
Table 2.
As is understood from the results, a difference between the charge
quantities in the HH condition and in the NN condition was as small as 0.3
.mu.C/g. Similarly, a difference between the charge quantities in the LL
condition and in the NN condition was also as small as 0.3 .mu.C/g. It was
thus confirmed that the carrier H was not changed in the charging
characteristics despite a change in ambient conditions.
Example 7
A carrier J was obtained in the same manner as in Example 6 except that the
treating time required for applying a chemical impact against the magnetic
powder was altered from 3 hours to 1 hour and the treating temperature was
altered from 70.degree. C. to 100.degree. C. The resulting carrier J was
observed using an SEM and was subjected to Auger electron spectrometry and
a measurement of charge quantity in the same manner as in Example 6.
As a result, it was confirmed that a magnetic powder having a octahedron
form was embedded in a polyethylene resin which is a coating layer and a
surface polyethylene coating layer was formed. It was also confirmed that
the thickness of the surface polyethylene coating layer formed on the
surface of the magnetic powder was 0.05 .mu.m. The charging
characteristics of the carrier J are shown in Table 2.
Example 8
10 kg of the carrier A.sub.2 was placed in a Henshel mixer which was the
same mixer that was used in Example 6. Hot water was flowed through a
jacket formed around the Henshel mixer to raise the temperature (treating
temperature) in the Henshel mixer to 70.degree. C. The Henshel mixer was
allowed to work to agitate the carrier for 0.5 hours while the temperature
was kept at 80.degree. C., thereby applying a mechanical impact to the
carrier to smooth the surface of the carrier A.sub.2.
Then, 200 g of a magnetic powder Magnetite MG1306 which was used in Example
6 and 120 g of hydrophobic silica R812 (manufactured by Nippon Aerogyl,
average diameter of primary particles: 0.02 .mu.m) were mixed and
thereafter the Henshel mixer was operated for four hours to apply a
mechanical impact, thereby forming a polyethylene resin layer containing a
magnetic powder and hydrophobic silica and a surface polyethylene coating
layer on the carrier A.sub.2. In addition, for the purpose of thoroughly
removing magnetic powder and hydrophobic silica which were not fixed and
remained in a free condition, though the amounts of these compounds were
small, the resulting product was treated in a screening process and a
classifying process in the same manner as in Example 6 to obtain a carrier
K.
The resulting carrier K was observed using an SEM and was subjected to
Auger electron spectrometry and a measurement of charge quantity in the
same manner as in Example 6.
As a result, it was confirmed that a magnetic powder having a octahedron
form and the hydrophobic silica were embedded in the outermost surface of
a coating layer to form an outermost layer and a surface polyethylene
coating layer was formed. It was also confirmed that the thickness of the
surface polyethylene coating layer was 0.01 .mu.m. The charging
characteristics measured of the carrier K are shown in Table 2.
Example 9
10 kg of the carrier A.sub.2 was placed in a Henshel mixer which was the
same Henshel mixer that was used in Example 6. Hot water was flowed
through a jacket formed around the Henshel mixer to raise the temperature
(treating temperature) in the Henshel mixer to 90.degree. C. The Henshel
mixer was allowed to work to agitate the carrier for 0.5 hours while the
temperature was kept at 90.degree. C., thereby applying a mechanical
impact to the carrier to smooth the surface of the carrier A.sub.2.
Then, 200 g of a magnetic powder Magnetite MG1306 which was used in Example
6 and had a octahedron form and 80 g of charging characteristic particles
MP2701 (PMMA, average particle diameter: 0.4 .mu.m, manufactured by Souken
Chemical Co., Ltd.) were mixed and thereafter the Henshel mixer was
operated for 3 hours to apply a mechanical impact, thereby forming a
polyethylene resin layer containing the magnetic powder and the charging
characteristic particles and a surface polyethylene coating layer on the
carrier A.sub.2. In addition, for the purpose of tho roughly removing the
magnetic powder and the charging characteristic particles which were not
fixed and remained in a free condition, though the amounts of these
compounds were small, the resulting product was treated in a screening
process and a classifying process in the same manner as in Example 6 to
obtain a carrier L.
The resulting carrier L was observed using an SEM and was subjected to
Auger electron spectrometry and a measurement of charge quantity in the
same manner as in Example 6.
As a result, It was confirmed that a magnetic powder having a octahedron
form and charging characteristic particles were embedded in the outermost
surface of a coating layer to form an outermost layer and a surface
polyethylene coating layer was formed. It was also confirmed that the
thickness of the surface polyethylene coating layer was 0.009 .mu.m. The
charging characteristics measured of the carrier L are shown in Table 2.
Example 10
A carrier M was obtained in the same manner as in Example 7 except that the
treating time required for mixing in the Henshel mixer after the magnetic
powder was mixed was altered from one hour to six hours.
The resulting carrier M was observed using an SEM and was subjected to
Auger electron spectrometry and a measurement of charge quantity in the
same manner as in Example 6.
As a result, it was confirmed that a magnetic powder having a octahedron
form and charging characteristic particles were embedded in the outermost
surface of a coating layer to form an outermost layer and a surface
polyethylene coating layer was formed. It was also confirmed that the
thickness of the surface polyethylene coating layer was 0.08 .mu.m. The
charging characteristics measured of the carrier M are shown in Table 2.
Example 11
A carrier N was obtained in the same manner as in Example 6 except that the
type of convex magnetic powder used in Example 6 was altered from the
Magnetite MG1306 (octahedron, average particle diameter: 0.2 .mu.m,
manufactured by Mitsui Mining & Smeling Co., Ltd.) to a Magnetite MG9300
(polyhedron, average particle diameter: 0.2 .mu.m, manufactured by Mitsui
Mining & Smeling Co. , Ltd.).
The resulting carrier N was observed using an SEM and was subjected to
Auger electron spectrometry and a measurement of charge quantity in the
same manner as in Example 6.
As a result, it was confirmed that a magnetic powder having a polyhedron
form was embedded in the outermost surface of a coating layer to form an
outermost layer and a surface polyethylene coating layer was formed. It
was also confirmed that the thickness of the surface polyethylene coating
layer was 0.007 .mu.m. The charging characteristics measured of the
carrier N are shown in Table 2.
Example 12
A carrier P was obtained in the same manner as in Example 6 except that the
type of convex magnetic powder was altered from the untreated Magnetite
MG1306 used in Example 6 to a Magnetite MG1306 which was treated
hydrophobically using a KBM703 (.gamma.-chloropropyltrimethoxysilane,
manufactured by Shin-Etsu Chemical Co., Ltd.) which was a silane coupling
agent.
In the hydrophobic treatment using a silane coupling agent, firstly a
solution in which the silane coupling agent was diluted and dissolved in a
mixture solution of water/alcohol was prepared. Then, the solution and a
magnetic powder were placed in a universal mixing stirrer and stirred at
80.degree. C., followed by drying. The magnetic powder which was
surface-treated using a dipping method in this manner was crushed using a
crusher to obtain a hydrophobically treated magnetic powder having a
convex form.
The resulting carrier P was observed using an SEM and was subjected to
Auger electron spectrometry and a measurement of charge quantity in the
same manner as in Example 6.
As a result, it was confirmed that a hydrophobically treated magnetic
powder having an octahedron form and the charging characteristic particles
were embedded in the outermost surface of a coating layer to form an
outermost layer and a surface polyethylene coating layer was formed. It
was also confirmed that the thickness of the surface polyethylene coating
layer was 0.005 .mu.m. The charging characteristics measured of the
carrier P are shown in Table 2.
Example 13
A carrier Q was obtained in the same manner as in Example 6 except that the
type of convex magnetic powder was altered from the untreated Magnetite
MG1306 used in Example 6 to a Magnetite MG1306 which was treated
hydrophobically using a SH1107 (methyl hydrogen silicon oil, manufactured
by Toray-Dow Corning Co., Ltd.) which was a silicon oil. The hydrophobic
treatment using silicon oil was carried out using a dipping method in the
same manner as in Example 12. The resulting magnetic powder was crushed to
obtain a hydrophobically treated magnetic powder.
The resulting carrier Q was observed using an SEM and was subjected to
Auger electron spectrometry and a measurement of charge quantity in the
same manner as in Example 6.
As a result, it was confirmed that a hydrophobically treated magnetic
powder having an octahedron form and the charging characteristic particles
were embedded in the outermost surface of a coating layer to form an
outermost layer and a surface polyethylene coating layer was formed. It
was also confirmed that the thickness of the surface polyethylene coating
layer was 0.005 .mu.m. The charging characteristics measured of the
carrier Q are shown in Table 2.
Example 14
10 kg of the carrier A.sub.2 was placed in a Henshel mixer which was the
same mixer that was used in Example 6. Low temperature water
(20-25.degree. C.) was flowed through a jacket formed around the Henshel
mixer to control the temperature (treating temperature) in the Henshel
mixer at 40.degree. C. The Henshel mixer was allowed to work to agitate
the carrier for 0.5 hours while the temperature was kept at 40.degree. C.,
thereby applying a mechanical impact to the carrier to smooth the surface
of the carrier A.sub.2.
Then, 200 g of a magnetic powder Magnetite MG1306 which was used in Example
6 was mixed and thereafter the Henshel mixer was operated for 3 hours to
apply a mechanical impact, thereby forming a polyethylene resin layer
containing a magnetic powder on the carrier A.sub.2. In addition, for the
purpose of thoroughly removing the magnetic powder which was not fixed and
remained in a free condition in a relatively large amount, the resulting
product was treated in a screening process and a classifying process in
the same manner as in Example 6 to obtain a carrier R.
The resulting carrier R was observed using an SEM and was subjected to
Auger electron spectrometry and a measurement of charge quantity in the
same manner as in Example 6.
As a result, it was confirmed that though, as shown in FIG. 9, a magnetic
powder having a octahedron form was embedded in the outermost surface of a
coating layer to form an outermost layer, it was exposed from the coating
layer and melt-fixed and no surface polyethylene coating layer was formed.
Moreover, the charging characteristics of the carrier R were measured in
the same manner as in Example 6. The results are shown in Table 2.
Comparative Example 3
The charging characteristics of the carrier A.sub.2 were measured in the
same manner as in Example 6. The results are shown in Table 2.
TABLE 2
Magnetic Temp. Time Thick.* LL NN HH
Carrier Powder (.degree. C.) (Hrs) (.mu.m) (.mu.C/g)
(.mu.C/g) (.mu.C/g)
Example 6 H Octa- 70 3 0.005 13.1 12.8 12.5
Hedron
Example 7 J Octa- 100 1 0.05 15.1 14.4 14.1
Hedron
Example 8 K Octa- 80 4 0.01 25.4 23.9 23.3
Hedron
Example 9 L Octa- 90 3 0.009 8.2 7.8 7.7
Hedron
Example 10 M Octa- 100 6 0.08 23.4 12.4 11.8
Hedron
Example 11 N Poly- 70 3 0.007 14.0 13.6 13.2
Hedron
Example 12 P Octa- 70 3 0.005 16.2 16.0 16.0
Hedron
Example 13 Q Octa- 70 3 0.005 9.1 8.8 8.7
Hedron
Example 14 R Octa- 40 3 0 13.4 13.0 6.9
Hedron
Comparative A.sub.2 -- -- -- 0 18.5 15.2 4.8
Example 1
*"Thick" stands for the thickness of the surface PE layer.
Examples 15 and 16 and Comparative Example 4
To each of the carriers H, R and A.sub.2 obtained in Examples 6 and 14 and
Comparative Example 1 respectively was added a toner (TK-12 model,
manufactured by Kyocera Co.) in a manner that the amount of the toner to
be added was 5% by weight based on the total amount to form a developer
for electrophotography.
Next, the resulting developers for electrophotography were respectively
placed in a printer, into which an Ecosys FS-3550 (manufactured by Kyocera
Co.) was remodeled, to make an actual printing of 50,000 copies in the
condition that the bias potential (voltage) was fixed at 300 V while the
image density of a solid portion was measured at regular intervals by
using a Macbeth densitometer.
Incidentally, the printer was remodeled so that the surface potential of a
sensitive body and the bias potential of a magnet roller could be
controlled. The results are shown in FIG. 5.
Example 17 and Comparative Example 5
To each of the carriers H and A.sub.2 obtained in Example 6 and Comparative
Example 1 respectively was added a toner (TK-12 model, manufactured by
Kyocera Co.) in a manner that the amount of the toner to be added was 5%
by weight based on the total amount to form a developer for
electrophotography.
Next, the resulting developers for electrophotography were respectively
placed in the aforementioned printer to make an actual printing of 50,000
copies while the bias potential (voltage) was varied between 150 and 350 V
and the image density of a solid portion was measured using a Macbeth
densitometer. The results are shown in FIG. 6.
As is clear from FIG. 6, it is understood that, by embedding a magnetic
powder in a coating layer of a carrier for electrophotography and by
forming a surface polyethylene coating layer so as to coat the magnetic
powder, the bias potential (voltage) is proportional to the image density
and hence a reduction in the image density is lessened even in high bias
potential zones. It is thus possible to obtain a clear printing contrast
and stable image characteristics.
INDUSTRIAL APPLICABILITY
As described above, the present invention can provide a carrier for
electrophotography, which is excellent in durability and a charging
property, gives clearer light and shade in real printing than the
conventional one, and allows fine and free adjustment of static resistance
and charge quantity, and a developer using the carrier.
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